1. Field of the Disclosure
Embodiments disclosed herein relate generally to a process for converting oxygenates to olefins. In one aspect, embodiments disclosed herein relate to a process for converting methanol to olefins (MTO). In another aspect, embodiments disclosed herein relate to an MTO process including separating and recovering ethylene from an MTO reactor effluent. In yet another aspect, embodiments disclosed herein relate to an MTO process including using a hydrocarbon absorbent to separate and recover ethylene from an MTO reactor effluent. In still another aspect, embodiments disclosed herein relate to the separation and recovery of ethylene from an MTO reactor effluent at conditions to avoid substantial formation of N2O3.
2. Background
Limited availability and high cost of petroleum sources has led to the increased cost of producing basic commodity chemicals and their derivatives from such petroleum sources. As a result, various alternative competing technologies have been developed and commercially implemented in order to produce these chemicals from non-petroleum sources at a competitive cost.
One such technology involves catalytically converting methanol to olefins (MTO). Methanol is a readily available feedstock, which can be manufactured both from petroleum as well as non-petroleum sources, for example, by fermentation of biomass or from synthesis gas.
A typical MTO process, as disclosed in U.S. Pat. No. 4,499,327, which is hereby incorporated in its entirety, involves contacting methanol with a zeolite catalyst, such as an aluminosilicate, under conditions of temperature and pressure in order to produce light olefins, such as ethylene. Ethylene is an extremely valuable commodity chemical for producing various derivatives, such polyethylene, used in many commercial as well as consumer products and applications.
Before ethylene produced by an MTO process can be sold and used, it is necessary to employ a process which recovers the ethylene component in a desirable, ethylene rich stream by separating it from other components and impurities. For example, depending on the feedstock composition, the reaction conditions, and the extent of side reactions, an MTO effluent can contain other light olefins and diolefins, and light paraffins such as methane. In addition, one particular side reaction that can occur during the MTO process is formation of nitrogen oxides, NO and NO2, commonly referred to as NOx, from nitrogen and oxygen in any entrained air in or nitrogen fed to the MTO reactor system.
One process for the separating and recovering of ethylene from an MTO process effluent involves the use of flash stages and distillation at cryogenic temperatures, as described in U.S. Pat. Nos. 7,166,757 and 4,499,327. As described therein, the current state of the art ethylene recovery and separation processes which dominate the industry involve cryogenic boiling point separation of ethylene and methane at temperatures that may be lower than −90° C. The cryogenic separation can be very expensive due to both the capital cost of the specialized vessel metallurgy and refrigeration equipment, and the operating costs, including compression and cooling for the energy-intensive chill train.
The use of cryogenic temperatures during the processes for treating the MTO process effluent can result in unstable and potentially dangerous operating conditions. For example, the NOx present in the MTO process effluent can react to form N2O3. Further, it has been found that the N2O3 formation rate significantly increases with decreasing temperature, thus making a cryogenic process especially susceptible. N2O3 is a highly oxidative compound, which can form highly unstable and highly reactive gums upon contact with poly-unsaturated compounds, such as butadiene. Even at cryogenic temperatures and at concentrations in the ppb levels, such unstable gums can accumulate and cause dangerous runaway reactions and even explosions.
Accordingly, there exists a need for an improved method of treating an MTO process effluent to separate and recover ethylene and other valuable products that reduces the capital and operating costs and improves the operation safety and stability.